Today's vehicle emissions standards call for major reductions in the oxides of nitrogen (NOx) and particulate matter (PM) emissions from diesel and other lean burn engines. To help meet these standards, engine manufacturers have developed engine-based strategies (such as exhaust gas recirculation (EGR) and engine timing modifications), and suppliers have developed improved exhaust aftertreatment devices such as the diesel particulate filter (DPF), selective reduction catalyst (SCR) and lean NOx trap (LNT). These devices are also referred to herein as “emissions control devices” or ECDs.
The use of DPFs for PM control and SCRs and LNTs for NOx control, together with in-cylinder control methods, has reduced tailpipe emissions sufficiently to meet current requirements for heavy-duty vehicles. However, a major application difficulty shared by these aftertreatment devices is that they are temperature-sensitive with a finite temperature window for good operation.